This invention relates to a method of integrating chemical vapor deposition copper (Cu) with low-k fluorinated amorphous carbon (a-F:C) in single level and multilevel damascene structures, and more particularly, to a method of providing a thin layer of an adhesion promoter material, such as relatively hydrogen-free hydrogenated silicon carbide (SiC:H), between layers of silicon nitride (Si3N4) and a-F:C to enhance the adhesion and mechanical properties of the damascene structure.
The designers and makers of large scale integrated circuits continue to make ever-smaller devices which allow for greater speed and increased device packing densities. The size of individual features (e.g., the transistor gate length) on ultra-large-scale (ULSI) circuits is shrinking to less than 0.25 microns. The resultant increase in packing densities on semiconductor chips, and the associated increase in functionality, has greatly increased the number and density of interconnects on each chip.
Smaller on-chip devices, packed closer together, with increased functionality and complexity, require interconnects (lines, vias, etc.) which are smaller, more complex (e.g., more wiring levels), and more closely-spaced. The smaller sizes of the interconnects, which increases resistance, and closer interconnect spacing, leads to resistance-capacitance (RC) coupling problems including propagation delays and cross talk noise between inter-level conductors. As interconnect lines, both inter-level and intra-level, become smaller and more closely spaced, RC delays become an increasing part of the total signal delays, offsetting any speed advantage derived from smaller device sizes. RC delays thus limit improvement in device performance. Small conductor size increases the resistivity (R) of metal lines and smaller inter-line and inter-level spacing increases the capacitance (C) between lines. Use and development of lower-resistivity metals such as copper will continue to reduce the resistivity of interconnect lines. Capacitance can be reduced by employing lower dielectric constant (i.e., lower-k) dielectric materials.
Since capacitance (C) is directly proportional to the dielectric constant (k) of the interconnect dielectric, RC problems presented by ULSI circuits can be reduced if a low-dielectric-constant (low-k) material is used as the insulating material disposed between and around the inter-level and intra-level conductors (the dielectric being referred to herein as the xe2x80x9cinterconnect dielectricxe2x80x9d or the xe2x80x9cinterconnect dielectric materialxe2x80x9d). Industry is seeking a suitable replacement for silicon dioxide (SiO2), which has long been used as a dielectric in integrated circuits. Silicon dioxide has excellent thermal stability and relatively good dielectric properties, having a dielectric constant of around 4.0.But there is now a need for an interconnect dielectric material which is suitable for use in IC circuit interconnects and which has a lower dielectric constant than does SiO2.
After a long search for possible low dielectric constant materials to be used as an interconnect dielectric in ULSI circuits, the candidates have been narrowed down to a few, depending upon the desired application. One of the promising materials, which has been actively studied recently and has received considerable attention, is fluorinated amorphous carbon (a-F:C).
The dielectric constant of a-F:C films is lowered as the fluorine concentration in the material is increased. In the plasma enhanced chemical vapor deposition process (PECVD) process, the fluorine concentration of the films depends on the fluorine to carbon ratio in the discharge, which is established by the feed gas composition, RF power input, substrate temperature, and total pressure. The thermal stability is closely related to the degree of cross-linking among the polymer chains. The greater the degree of cross-linking, the more tightly bound the structures are, and the higher the thermal stability. In the PECVD process, either raising the substrate temperature, enhancing the ion bombardment, or applying a low frequency plasma energy, can increase the cross-linking in the fluorocarbon films. Higher temperature deposition has the disadvantage of inevitably reducing the fluorine concentration, thereby increasing the dielectric constant. Moreover, higher temperature deposition also leads to poor adhesion between the polymer layer and the SiO2 and Si3N4 layers due to increased thermal stress, and causes higher leakage current in the films.
Fluorinated amorphous carbon has a dielectric constant k below 3.0 and, depending on the proportion of fluorine (F) in the film, can have a dielectric constant in the range of 2.0 to 2.5. Early experience with these polymers shows that films deposited at room temperature may have a dielectric constant as low as 2.1 and thermal stability up to 300 C. Further experimentation showed that if the a-F:C films were deposited at higher substrate temperatures, the thermal stability could be improved up to 400 C., but the dielectric constant increased above xcx9c2.5. It has heretofore not been possible to prepare a-F:C films with suitable low-dielectric-constant properties (k less than 2.5), and a thermal stability above 400xc2x0 C. Temperatures in the sintering range (450xc2x0 C.), typical for manufacturing ULSI chips, cause excessive shrinkage of the a-F:C film, probably due to fluorine volatilization. Mechanical strength and adhesion problems also are obstacles to the use of a-F:C as an interconnect in high-density integrated circuits. In particular, the poor adhesion between a-F:C and the barrier layer, such as silicon nitride (Si3N4), has been a problem for years.
Recent research and development on low dielectric constant (low-k) materials indicates that the integration of Cu with low-k materials is one of the key issues in choosing candidates for future interlayer dielectrics in single-level and in multi-level damascene structures. Although many low-k candidates show excellent electrical properties, a successful Cu/low-k integration has still not been accomplished due to difficulties in the fabrication of Cu/low-k based damascene structures. In these structures, the major reliability issues are the adhesion of low-k films, such as a-F:C, with SiO2, Si3N4 and barrier layers (liners), the mechanical strength of the low-k materials during chemical mechanical polishing (CMP), and the stability of the single and multi-level damascene structures under heat treatment, patterning and plasma etching. Since multilevel wiring is the ultimate goal for the Cu/low-k interconnection, fabrication of such multi-level damascene structures is critical.
Accordingly, it would be advantageous to have a dielectric material, alternatively referred to herein as an xe2x80x9cinterconnect dielectric,xe2x80x9d for use in interconnect structures of integrated circuits which has a low dielectric constant (k=3.0 or less) and improved thermal stability (up to 450xc2x0 C.), thus providing a suitable lower-k replacement for silicon dioxide dielectric.
It would also be advantageous to have a a-F:C film which has a dielectric constant of 2.5 or less which is thermally stable to 450xc2x0 C.
It would also be advantageous to have a method of forming low-k a-F:C films on silicon substrates using plasma enhanced chemical vapor deposition (PECVD) techniques, wherein the resultant a-F:C film is substantially stable up to 450xc2x0 C.
It would further be advantageous to have a method of forming multiple a-F:C films on silicon substrates wherein the resultant multiple layered a-F:C/Si3N4 structure is stable up to 450xc2x0 C.
It would be advantageous to have a method of forming a thin layer of promoter material, such as SiC, to enhance the adhesion and mechanical properties of a-F:C films with SiO2 and/or Si3N4 layers.
It would be advantageous to have a method of forming a promoter layer which may also serve as a barrier to reduce diffusion of fluorine atoms through the layered structure.
It would be advantageous to have a method of forming Cu/SiO2/a-F:C damascene stacking layers that can sustain the processes of CMP, heat treatment, patterning and plasma etching.
A plasma enhanced chemical vapor deposition (PECVD) process is provided for depositing one or more dielectric material layers on a substrate for use in interconnect structures of integrated circuits. The method comprises the steps of depositing a fluorinated amorphous carbon (a-F:C) layer on a substrate by providing a fluorine containing gas, preferably octafluorocyclobutane, and a carbon containing gas, preferably methane, in a ratio of approximately 5.6, so as to deposit a a-F:C layer having an internal compressive stress of approximately 28 MPa. After deposition the film is annealed at approximately 400xc2x0 C. for approximately two hours and results in the layer having an internal tensile stress of approximately 30 MPa.
An adhesion promoter layer of relatively hydrogen free hydrogeneated silicon carbide is then deposited on the a-F:C layer by providing a silicon containing gas, preferably silane, and a carbon containing gas, preferably methane, in a flow ratio of approximately 0.735. The deposition typically takes place at a pressure of approximately 2.4 Torr, a high frequency power of 200 Watts and 13.56 MHz, a low frequency power of 200 Watts and 500 KHz, and a temperature of approximately 400xc2x0 C. The silicon carbide layer may be deposited at a rate of approximately 180 xc3x85 per minute and typically results in deposition of a silicon carbide layer having an internal compressive stress of approximately 400 MPa. The conditions of the deposition result in a deposited silicon carbide layer that has relatively few silicon-hyrogen bonds thereby yielding a compact structure which promotes adhesion of the damascene structure layers to one another and which reduces diffusion of fluorine through the silicon carbide layer.
A silicon nitride layer is then deposited on the adhesion promoter layer, the deposition materials preferably comprising silane (SiH4) and nitrogen (N2) in a flow ratio of 0.539 at 400xc2x0 C. The silicon nitride layer formed has relatively few silicon-hydrogen bonds thereby resulting in a layer having an internal compressive stress of approximately 240 MPa. This stacked layer structure has thermal stability and resists peeling and cracking up to 450xc2x0 C., and the a-F:C layer has a dielectric constant as low, or lower, than 2.5.